1. Field of the Invention
The present invention relates to a process for producing isocyanates by thermal decomposition of carbamic acid esters. Isocyanates are useful as raw materials for producing polyurethane, polyurea, etc., and have been commercially produced on a large scale.
2. Prior Art
Isocyanates have been industrially produced, usually by the reaction of amines with phosgene, i.e., the so-called phosgene process. However, the phosgene process has problems in handling of phosgene having intensive toxicity, treatment of hydrochloric acid being by-produced in a large amount, corrosion for apparatuses, etc., and thus, instead of the phosgene process, development of an industrially distinguished process for production of isocyanates has been keenly desired.
As one process thereof, there is a process for thermally decomposing carbamic acid esters. It is known that isocyanates can be obtained by heating carbamic acid esters in a liquid phase. However, in the absence of a catalyst, a thermal decomposition rate is generally low, and when a thermal decomposition temperature is elevated, high boiling point by-products increase, so that a yield tends to decrease. Accordingly, as processes for ensuring a high thermal decomposition rate and suppressing side reactions, various processes for using a catalyst, a stabilizer, etc., have been proposed.
That is, a process for producing isocyanates by thermal decomposition of carbamic acid esters in the presence of a catalyst, for example, includes a process for using a catalyst dissolved in a solvent at least one metal thereof selected from the group consisting of metallic atoms belonging to IB, IIB, IIIA, IVA, IVB, VB and VIII groups in the periodic table of elements or a metallic compound thereof, as disclosed in U.S. Pat. No. 4,081,472. JP-A-54-88201 discloses a process for using an alkaline earth metal(s) and a metallic compound(s) thereof as a catalyst. JP-A-57-158747 discloses a process for using at least one of elements or compound selected from elements belonging to copper group, zinc group, aluminium group, carbon group except carbon, and titanium group in the periodic table of elements and oxides or sulfides thereof in a solvent as a heterogeneous catalyst.
U.S. Pat. No. 4,388,246, in order to suppress side reaction in which high boiling point by-products are produced, proposes a process for using a stabilizer selected from the group consisting of hydrogen chloride, organic acid chloride, substances having an alkylation function being composed of an alkyl ester of organic or inorganic acid and organic tin-(VI)-chloride in the presence or absence of the above-mentioned catalyst.
A process for producing isocyanates by thermal decomposition of carbamic acid esters (urethanes), even in case of using the above-mentioned catalyst or stabilizer, suffers from such disadvantages as much formation of high boiling point by-products or a low space-time yield.
Accordingly, it has been proposed to use a homogeneous or heterogeneous catalyst being composed a heavy metal(s) including Co, Mn, Fe, Ni, etc., or a metallic compounds(s) thereof. However, in such catalysts, a catalyst composition deposits along with high boiling point by-products with catalyst's deterioration or deactivation, etc., to cause adhesion or sedimentation on the inside of a reactor, so that it becomes difficult to ensure a long-term stationary operation. Further, these catalysts after use are separated along with high boiling point by-products from a solvent in an evaporator, a filter, etc. A treatment for the separated by-products is complicated because a heavy metal(s) is contained in the by-products.
The object of the present invention is to provide a process for producing isocyanates by thermal decomposition of carbamic acid esters in which a high thermal decomposition rate is ensured and side reaction wherein high boiling point products are produced is suppressed and furthermore isocyanates can be stationarily produced in a high space-time yield for a long term.